1. Technical Field of the Invention
The present invention relates generally to a gas sensor element which may be built in a gas sensor employed in an air-fuel ratio control system for automotive vehicles for measuring the concentration of gas such O2, NOx, or CO, and more particularly to a nonfragile ad quickly activatable structure of a gas sensor element.
2. Background Art
Typical exhaust systems of automotive engines use a gas sensor for air-fuel ratio control. As such, there are known ones which consist of a solid electrolyte body having oxygen-ion conductivity, a measurement gas electrode exposed to a gas to be measured, and a reference gas electrode exposed to a reference gas. The measurement gas electrode and the reference gas electrode are installed on the solid electrolyte body. An oxygen-ion current is produced which flows through the measurement and reference gas electrodes and is used for determining the air-fuel ratio.
In recent years, there is an increasing need for speeding up an elevation in temperature of a sensor element of the gas sensor and/or reducing the size of the gas sensor in order to shorten the active time required for the gas sensor to start to produce a correct output and enable the gas sensor to be mounted in various places (e.g., in an exhaust pipe beneath a floor of a vehicle body).
To meet the above requirements, gas sensors equipped with a heater have become employed.
In recent years, the activation time of gas sensors is required to be shortened further in order to enhance the performance of a three way catalytic converter to convert polluting exhaust gasses into harmless products immediately after the startup of the engine.
However, an increase in temperature of the heater for achieving the quick activation of the sensor element will cause a great thermal stress to act on a portion of a body of the sensor element near the heater, thus resulting in formation of cracks in the body of the sensor element.
In order to avoid the above drawback, Japanese Patent First Publication No. 2-75188 teaches a heater which is so designed as to meet a relation of 0.7Xxe2x89xa6Yxe2x89xa61.5X where X is a minimum distance between a side of a heater substrate and a side of a heater element, and Y is a minimum distance between a tip of the heater substrate and a tip of the heater element. Use of a heater having such dimensions serves to decrease cracks in a top portion of the sensor element, but does not avoid cracks in side portions thereof.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a gas sensor element which is capable of being activated fast and nonfragile in structure.
According to one aspect of the invention, there is provided a gas sensor element which may be built in a gas sensor for measuring the concentration of gas such O2, NOx, or CO used in an air-fuel ratio control system of automotive vehicles. The gas sensor element comprises: (a) a solid electrolyte member having opposed surfaces; (b) a measurement gas electrode disposed on one of the opposed surfaces of the solid electrolyte member, the measurement gas electrode being exposed to a gas to be measured; (c) a reference gas electrode disposed on the other of the opposed surfaces of the solid electrolyte member, the reference gas electrode being exposed to a reference gas; and (d) a heater laminated on the solid electrolyte member. The heater includes a heater substrate, a heating element, and leads for supplying power to the heating element. The heating element and leads are disposed on the heater substrate. A minimum distance X between an edge of the heater substrate and an edge of the heating element is so set as to meet a relation of 0.1 mmxe2x89xa6Xxe2x89xa60.6 mm.
If the distance X is less than 0.1 mm, it becomes difficult to ensure an interval between the heating substrate and the heating element required for establishing desired electric insulation therebetween. This causes the current flowing through the heating element to leak to the measurement gas and reference gas electrodes and a portion of the solid electrolyte member near the electrodes, which may result in an error in measuring the concentration of the gas. Alternatively, if the distance X is greater than 6.0 mm, it may result in an increase in thermal stress when the gas sensor element is heated quickly which causes cracks to occur in, for example, the heater substrate.
In the preferred mode of the invention, the heater substrate and the heating element each have a length. A minimum distance Y between a tip of the heater substrate and a tip of the heating element is so set as to meet a relation of 1.0 mmxe2x89xa6Yxe2x89xa62.5 mm. This serves to decrease a difference in temperature or thermal expansion between the heating element and a portion of the heater substrate near the heating element, thereby ensuring a desired resistance to cracks. It also becomes possible to elevate the temperature of the heating element up to a higher value, thereby allowing the gas sensor element to be activated quickly after the power is supplied to the heating element.
A reference gas chamber is defined in the gas sensor element into which the reference gas is admitted and to which the reference gas electrode is exposed. The reference gas chamber has a length substantially coinciding with a length of the heating element, and a width A of the reference gas chamber and a width B of the heating element meet a relation of A less than B. This dimensional limitation allows a projected area of the reference gas chamber to be included in a projected area of the heating element, thereby resulting in ease of transmission of heat from the heating element to the reference gas chamber. This decreases a difference in temperature between the measurement gas electrode, the reference gas electrode, and a portion of the solid electrolyte member near the electrodes, thereby avoiding occurrence of cracks in the gas sensor element.
The gas sensor element has a width of 3 mm to 6 mm. This results in an increased resistance of the gas sensor element to cracks due to thermal shocks and enables the pattern of the heating element which is excellent in heating efficiency to be designed.